You have the option to have your navigation bar appear either before your page content starts, or to sit directly on top of it.
The 'Straight on Top' option is ideal when you are not using a Cover section but you want your navigation bar to be transparent, or perhaps when you don't want excessive white space above your page content.
To set your navigation bar to appear before page content, go to your page branding options and set the "Display Content" option to "After Navigation".
To set your navigation bar to appear on top of your page content, set the "Display Content" option to "Straight on Top".
Page content set to appear after navigation:
Navigation bar set to appear straight on top, with transparent background: